Hi. New member here.
Any ideas on a brand new "hybrid" bike with 18 speed Shimano Tourney gears fitted which is occasionally throwing the chain off the smallest (high gear) rear sprocket?

The stops are adjusted correctly as far as I can see, or at least so that the chain will only just go over onto the smallest sprocket from 5th. No cable tension as such in that gear, but no slack. Stops are moving the arm. Gears generally seem to change OK. There doesn't seem to be any slackness or anything in the arm, and idlers. The arm seems straight. Front gear was on the centre chain ring.

The chain came off the other night, and I looked at it today expecting the adjustment to have moved, but it was still right. This has happened a couple of times

It won't do it pedalling on a stand. It works fine.

No real experience with derailleurs, but years of experience with bikes, cars, motorbikes, and worked with electrical and mechanical equipment at one time.

Thanks for the reply. I've been careful not to shift under load. They're indexed gears on a twistgrip, and again I've been trying to change one gear at a time.
Can't see that the arm is bent. We have two identical bikes apart from frames, and the one has been OK after initial adjustment. They both look the same. Neither have been dropped or knocked. They really haven't been used much with the weather!
Looks like it might be a faulty part. I could chase the warranty I guess. Should be 12 months here in the UK. They were bought mail order off eBay about three months ago. I'll have a word with them and see what they say.
I think I'm right in saying that the arm for this model is fairly inexpensive if I need to replace it myself?